A report indicates that over half of the iPhone apps featuring ads had less than 1000 users in May.

The report by mobile ad network AdMob shows that 54% of the 2,300 apps in which ads appear had fewer than 1,000 users - compared to the most popular apps, 5% of the market, which had more than 100,000 users.

The biggest hits had more than 2 million, and about 11 apps had over 1 million users.

A low-end, ad-supported iPhone app can expect to make a net eCPM around $1-$2 - meaning that for every 1,000 times an ad loads on the app, this yields $1-$2 after sharing revenue with the ad network. Assuming 1,000 people will look at an estimated 10 ads per month, one can expect to bring in just $10-$20 in revenue.

More popular apps with hundreds of thousands of users obviously fare better.

The report only covers apps using AdMob's ads and does not necessarily reflect the broader iPhone app market.

Yesterday Google launched AdSense for Mobile Apps, which lets users develop programs for iPhone and Android.

Last month, a study found that nearly four in 10 (38%) US mobile phone owners, and 59% of iPhone owners recall seeing advertising on their phones in Q109.

Another report released in May indicated that the average iPhone app has an engagement of 9.6 minutes per session and is accessed 19.9 times over its life cycle.